Competing fracture modes in brittle materials subject to concentrated cyclic loading in liquid environments: Bilayer structures

A preceding study of the competition between fracture modes in monolithic brittle materials in cyclic loading with curved indenters in liquid environments is here extended to brittle layers on compliant substrates. The fracture modes include outer and inner cone cracks and radial cracks that initiate from the near-contact zone and penetrate downward. Outer cone cracks are driven by stresses from superposed Hertzian and plate flexure fields; inner cone cracks also grow within these fields but are augmented by mechanical driving forces from hydraulic pumping into the crack fissures. Radial cracks are augmented by mechanical driving forces from developing quasiplasticity zones beneath the indenter. Basically, the crack-growth rates are governed by a crack velocity relation. However, the hydraulic and quasiplastic mechanical forces can cumulate in intensity with each cycle, strongly enhancing fatigue. Plate flexure generates compressive stresses at the top surface of the brittle layer, somewhat inhibiting the initiation, and tensile stresses at the lower surface, strongly enhancing the far-field propagation. The tensile stresses promote instability in the crack propagation, resulting in through-thickness penetration (failure). Experiments on a model bilayer system consisting of glass plates bonded to thick polycarbonate bases are presented as an illustrative case study. In situ observations of the crack evolution from initial growth to failure reveal that each fracture mode can dominate under certain test conditions, depending on plate thickness, maximum load, and sphere radius. Implications concerning the failure of practical layer systems, notably dental crowns, are discussed.